Many different methods are available for pumping solid state lasers, such as neodymium doped yttrium aluminum garnet (Nd:YAG) lasers. One common technique is to place a rod of solid state material at one focus of a tubular reflector having an elliptical cross section and a flash lamp or other bright light source at the other focus. In such an arrangement, light emitted by the flash lamp and reflected from the reflector walls will impinge on the rod. One problem with this arrangement is that the rod must have a diameter large enough to absorb a substantial portion of the pumping radiation during passage through the rod. If during this initial traverse the pumping illumination is not absorbed, it is likely to be reflected by the reflector walls back to the light source, where it will be reabsorbed, generating heat and reducing the lifetime of the source. Another problem is that much of the optical energy produced by flash lamps and other broadband light sources is wasted, because it does not match the absorption spectrum of the laser medium.
In U.S. Pat. No. 3,982,201, Rosenkrantz et al. disclose a solid state laser in which a Nd: YAG laser rod is end pumped by an array of semiconductor laser sources. The wavelength of the pump light from the diode lasers is selected for optimum absorption, while the increased optical path length of the pump light within the rod from end pumping relative to that from side pumping ensures more complete absorption. The lasers are cryogenically cooled and operate in a pulsed mode with a low duty cycle to enable heat generated by the diodes to dissipate between pulses and thereby maintain the array at a temperature which provides the desired pump wavelength.
Individual diodes and diode lasers, as well as arrays of diode lasers just described, have been directly coupled to the end of solid state rods to achieve low to medium power laser output. However, the power outputs of these solid state lasers are limited by the brightness of the pump light. In order to achieve higher power, brighter pump sources are required along with efficient means for coupling light from these sources into the solid state medium.
In U.S. Pat. No. 4,575,854, Martin discloses a Nd:YAG laser which is side pumped by a plurality of diode laser bars. Each bar contains many diode lasers which in turn circumferentially envelope a Nd:YAG rod or other suitable solid state medium. The bars are driven by a high frequency pulse which is switched so as to drive the bars in any desired combination, but not all at the same time. The laser operates in a continuous wave mode even though any given laser bar is pulsed at a very low duty factor so that the array may operate uncooled.
Diode laser bars with 30 W peak power output, a 50 Hz repeat rate and a 150 .mu.sec pulse length have been demonstrated, and it would be desirable to use laser bars to end pump a solid state laser. However, if one were simply to butt couple a diode laser bar to the end of a solid state rod, the laser would not be efficiently pumped, because of the elongated diode laser bar geometry. Laser bars may have a lateral dimension or width of up to 1 cm, which is too great to form a small enough image to fit within the fundamental mode volume of a solid state rod. Typical rods are 3 mm in diameter and have a fundamental mode volume about 100 .mu.m in diameter.
In U.S. Pat. No. 4,653,056, Baer et al. disclose an intra-cavity frequency-doubled Nd:YAG laser which allows efficient coupling by a high power laser diode array, despite the fact that the diode array has an output beam with too much spatial structure and limited focusability. Baer et al. achieve this result by expanding the lasing volume to match the focused image of the laser diode array. A combination of a concave output coupler mirror and a lens-shaped end at the front end of the Nd:YAG rod enables the beam size within the YAG rod to be adjusted to the appropriate volume. For efficient pumping, the pumping volume must overlap and preferably match closely the lasing volume of the rod.
It is not always possible or desirable to change the shape and size of the mode volume to match the pump light of a diode laser bar. Further, it may be desired to further increase the power by using a plurality of diode laser bars for pumping, as taught for example by Martin. Thus, ways of producing a bright source that fits the available mode volume of a solid state laser are sought. It may also be desired to operate a plurality of diode laser bars continuously, but still provide the necessary heat dissipation.
An object of the present invention is to produce an optical pumping system for end pumping a solid state laser.